The invention relates to direct access data storage devices such as magnetic disc storage devices. In a magnetic disc storage device, a magnetic data storage disc spins at a high speed while a transducer (or head) is suspended slightly above the disc surface. The transducer flies above the disc surface on an air bearing. The transducer typically includes a magnetoresistive read element and an inductive write element on the slider, commonly called the head. The head writes information onto the disc in concentric circular tracks. Different tracks are accessed for writing or reading by changing the radial position of the recording head so as to position the head above the desired track.
One of the most important parameters in a high-density magnetic disc storage device is the height (the fly height) of the read and write elements on the recording head above the disc surface while the disc is spinning. Rapid and accurate measurement of this quantity is essential for the development of new slider designs. Such fly height measurement is also essential quality control during slider manufacture after the development of new slider designs.
It has been known, in general, to measure the fly height of a magnetic recording head by white light interferometry between a spinning transparent test disc and a slider flying above the test disc. While this technique is sufficiently precise for the current generation of products, the need to measure fly heights to accuracies of a fraction of a wavelength of light threatens to push past the limits of this measurement technique.
In another method of fly height measurement, the slider flies above the surface of an electrically conductive test disc. By measuring the capacitance between the electrically conductive slider and the electrically conductive disc, the fly height can be determined. The known capacitance measurement technique suffers several drawbacks. Since the entire test disc is conductive, in order to be accurate this technique requires careful accounting for stray capacitance. Moreover, this measurement technique provides only an average fly height measurement. It provides no detailed measurement of the fly height at different locations on the slider. For example, the tilt of the slider with respect to the disc cannot be measured by the known capacitance technique. The tilt of the slider is important in order to determine the fly height of the trailing edge of the slider. The fly height of the trailing edge of the slider is important because the trailing edge of the slider is closest to the disc, and because the read and write elements of the head are located on the trailing edge of the slider. Further since the known capacitance measurement technique cannot determine the tilt of the slider, only the relative fly height can be determined. The absolute fly height cannot be determined by measuring the total capacitance between the slider and the conductive disc surface. Accordingly, improvements are desired.